Single point mooring systems have frequently been used in offshore locations for loading and unloading hydrocarbons or other flowable cargos into or out of marine vessels such as tankers, barges or the like. Many such systems have been developed and are now in use for both loading and unloading hydrocarbons at such offshore locations. Examples of such conventional systems include the catenary anchor leg mooring (CALM) apparatus and single anchor leg mooring (SALM) apparatus.
The catenary anchor leg mooring system (CALM) holds a floating buoy by an array or pattern of anchors attached by mooring chains or lines. The barge or tankers are attached to the floating buoy by suitable mooring ropes or a rigid mooring arm if desired. In such an anchor system, the position restoring mooring forces are provided by the horizontal force component of the mooring chains. However, in shallow water, the mooring chains cannot be arranged with sufficient slack to provide adequate resiliency or horizontal spring in the mooring system. In such instances, the horizontal movement of a moored floating vessel caused by dynamic external forces due to wave, wind and tidal currents can cause sudden extremely large forces which can exceed the capacity of the mooring system to maintain the vessel in the hydrocarbon transferring position. Because of the shallow water it was difficult to obtain the desired mooring restoring capacity to withstand these large dynamic forces without incurring the expense of additional anchors and mooring chains.
The "underwater buoy hoses" used to compensate for the vertical and horizontal motion of the buoy have been subjected to excessive curvature and bending or flexing in shallow water installations. This additional bending not only shortens the useful life of the expensive hydrocarbon transfer hose, but it also increases the risk of damage or hose failure with the attendant pollution of the water and loss of valuable hydrocarbons. Since the buoy itself floats on the sea surface, it has also been vulnerable to full impact from any passing ice floes. Therefor, the conventional CALM system is not at all suited for use in regions having large masses of floating ice.
The conventional single anchor leg mooring (SALM) uses or employs a single floating mooring buoy which is attached with a single chain or articulated arm to a lower base structure fixed to the sea bottom or floor. By submerging the buoy to a certain depth, a desired tension level is created in the anchor leg. Such tension provides a constant restoring or horizontal urging force for resiliently mooring a floating vessel to the buoy. In shallow water, the vessel position restoring capacity of the SALM mooring system is also reduced and not suitable for mooring large vessels.
Likewise, the conventional SALM system was also not particularly well suited for use in a location or region where large flow ice floes may contact or can be expected to impact the mooring buoy.